The present invention particularly concerns an and apparatus for treating a gaseous effluent from a Claus plant or a gas containing hydrogen sulphide and sulphur dioxide.
In particular, it concerns the treatment of effluents from Claus plants from hydrodesulphurization and catalytic cracking units. It also concerns the purification treatment of natural gas.
The prior art is illustrated by French patent application FR-A-2 411 802.
The Claus process is widely used to recover elemental sulphur from gaseous feeds containing hydrogen sulphide (H.sub.2 S). However, the fumes emitted from these Claus type plants, even after several catalytic stages, contain non negligible amounts of acid gases. Those effluents (tail gases) from Claus plants must, therefore, be treated to eliminate the majority of toxic compounds so as to satisfy anti-pollution regulations. These regulations are becoming more and more strict and existing technology must be constantly improved.
As an example, about 95% by weight of the sulphur present can be recovered from a Claus plant; treatment of this Claus plant tail gas (using a Clauspol unit, for example) can recover 99.8% by weight of the sulphur, for example, using the reaction: EQU 2H.sub.2 S+SO.sub.2 {character pullout}3S+2H.sub.2 O
which uses a reaction medium constituted by an organic solvent and a catalyst comprising an alkaline or alkaline-earth salt of an organic acid. The reaction is generally carried out in counter-current mode in a reactor-contactor and its temperature is controlled by passing the solvent which is extracted from the lower end of the reactor by a circulating pump through a heat exchanger to encourage the highest possible degree of conversion to sulphur while avoiding the formation of solid sulphur. Sulphur is thus recovered in liquid form. While it is very effective, the process is limited by various constraints:
The thermodynamic equilibrium of the reaction is such that the reaction is never complete. Some hydrogen sulphide and sulphur dioxide remains, in equilibrium with the sulphur and water which are formed. The quantity of sulphur present in unreacted H.sub.2 S and SO.sub.2 which is found in the reaction effluent (from the Clauspol) corresponds to about 0.1% of the total sulphur in the initial feed to the Claus plant. Better conversion can be envisaged at a lower operating temperature but this temperature must be kept above the freezing point of sulphur (about 120.degree. C.) otherwise the reactor will be blocked with solid sulphur; PA1 The presence of unseparated liquid sulphur in the reactor-contactor, which is entrained in the solvent and catalyst which circulate, and which is recycled to the reactor-contactor. Not all of the droplets of liquid sulphur are separated from the solvent and the presence of liquid sulphur irremediably causes the presence of gaseous sulphur in the effluent due to the vapour pressure of sulphur. As an example, the quantity of unrecovered sulphur which can be attributed to vapour pressure is about 0.1% by weight of the sulphur in the initial feed. PA1 In a first variation, if the organic solvent is miscible with water, it can be cooled by heat exchange in a heat exchanger before being mixed with the gaseous effluent to be purified, by adding water at a temperature which is lower than that of the organic solvent, wherein the heat of vaporisation on contact with the gaseous effluent can reduce the temperature of the mixture, or by a combination of the above two steps. Cooling is preferably by injection of water. PA1 In a second variation, if the organic solvent is not miscible with water, it can be cooled in the same manner as in the first variation. Cooling is preferably by heat exchange. PA1 In the category of solvents which are insoluble in water are hydrocarbons with boiling points at atmospheric pressure of more than 200.degree. C., preferably dodecane, tridecane, and naphtha with boiling points in the range 225.degree. C. to 335.degree. C. PA1 In the category of solvents which are soluble in water, with boiling points at atmospheric pressure of more than 200.degree. C. are polyols containing 2 to 15 carbon atoms, preferably glycerol, thiodiglycol and cyclohexanedimethylethanol, acid esters containing 5 to 15 carbon atoms, more particularly trimethylpentanediol mono-isobutyrate and dimethyl adipate, glycol ethers containing 5 to 15 carbon atoms, advantageously butoxytriglycol, ethoxytriglycol, diethylene glycol butylether, ethylene glycol phenylether, terpinyl ethylene glycol monobenzyl ether, ethylene glycol butylphenylether, diethylene glycol, diethylene glycol dimethylether, diethylene glycol dibutylether, triethylene glycol, tetraethylene glycol dimethylether, propylene n-butylether, dipropylene n-butylether, tripropylene n-butylether, and polyethylene glycol with a molar mass of 200, 300, 400 or 600.
The aim of the invention is to overcome the disadvantages of the prior art.
A further aim of the invention is to satisfy the strictest regulations designed to counter atmospheric pollution by sulphur-containing compounds.
A yet still further aim of the invention is to be able to modify existing installations with a Claus plant and a unit for treating the effluents from that unit (a Clauspol unit), at very low cost.
It has been shown that by eliminating all of the sulphur vapour from the effluents from gas treatment units, for example Claus plant tail gas, up to 99.9% of the total sulphur can be recovered and thus the quantity of sulphur discharged into the atmosphere when the gas is incinerated can be minimised.
More precisely, as aspect of the invention concerns a process for the treatment of a gas containing hydrogen sulphide and sulphur dioxide, in which the gas is brought into contact with an organic solvent containing a catalyst in at least one gas-liquid reactor-contactor at a suitable temperature, and a gaseous effluent which substantially no longer contains hydrogen sulphide and sulphur dioxide but which contains sulphur in vapour form is recovered, the process being characterized in that the gaseous effluent from the reactor-contactor is brought into contact with the same organic solvent or with another organic solvent at a temperature which is lower than the solidification temperature of sulphur (for example 95.degree. C.) in a contactor-cooler.
We have observed that bringing an organic solvent which is partially depleted in sulphur into contact with a gaseous feed from which a portion of the H.sub.2 S and SO.sub.2 has been removed has produced very good results.
In more detail, in the process for the treatment of a gas containing hydrogen sulphide and sulphur dioxide the gas (3) is brought into contact with at least one organic solvent in a first gas-liquid contact and reaction zone at a suitable temperature and an effluent containing water and sulphur vapour is recovered separately from an effluent containing sulphur, the process being characterized in that the gaseous effluent is introduced into a second contact zone, and brought into contact under suitable conditions with at least one recycled organic solvent which is depleted in sulphur, a purified gas substantially no longer containing sulphur vapour is recovered separately from said solvent which is rich in sulphur, at least a portion of said sulphur-rich solvent is removed, advantageously at most 50% of the flow, said portion of solvent is cooled to obtain a suspension of sulphur crystals in the solvent, the sulphur crystals are separated from the solvent and said portion of cooled solvent which is depleted in sulphur is recycled at least in part to the second contact zone, the process being further characterized in that at least one of the two contact zones contains at least one catalyst.
In accordance with one characteristic of the process aspect invention, the remaining portion of the sulphur-rich organic solvent from the second contact zone is recycled to the second contact zone, more precisely to its upper portion, after optional cooling.
In a further characteristic of the process, the portion of solvent which is to be depleted in sulphur is cooled by indirect heat exchange or by mixing with a suitable quantity of water or by a combination of these means at a temperature which is generally less than the melting temperature of sulphur, preferably at a temperature which is in the range 40.degree. C. to 110.degree. C. The quantity of water which is advantageously introduced is such that a solvent/water mixture containing 30% to 70% by weight of water is obtained.
The sulphur depletion operation consists of removing at least a portion of the sulphur-enriched organic solvent, generally at most 50% of the flow leaving the second contact zone and preferably 2% to 10% of the flow of liquid phase, and cooling it to a temperature such that a suspension of sulphur crystals is obtained in the solvent saturated in sulphur at said cooling temperature. After separating the sulphur crystals, the solvent which is depleted in sulphur with respect to that present in the second reactor-contactor can be re-heated to the temperature of the second reactor-contactor before being introduced into it.
At least a portion of a single-phase solution of said organic solvent can be extracted from the lower portion of the first contact zone, cooled to eliminate at least a portion of the heat of reaction released, then recycled to the first contact zone.
The temperature of the second contact zone is advantageously less than that of the first zone, preferably by 15.degree. C. to 20.degree. C.
It is of great advantage to use the same organic solvent in the first and in the second contact zones. In this case, a solvent line can be connected between the means for recycling the single-phase solution with reduced temperature from the first reactor, and the inlet to the heat exchanger cooling at least a portion of the sulphur-enriched solvent. This line acts as the line for adding solvent to the second reactor-contactor.
The catalyst is preferably introduced into the first reactor-contactor. Thus the majority of the sulphur present in the form of H.sub.2 S and SO.sub.2 and contained in the gas to be treated is eliminated, the second reactor only providing a finishing treatment which reduces the dimensions of the equipment. Clearly, it can be introduced into the second reactor-contactor alone, or into both.
For vertical reactors, there are two variations of the process of the invention.
In a first variation, both the gas and organic solvent in the first contact zone and the gaseous effluent and organic solvent in the second contact zone are brought into contact in co-current mode, the gas supply or the gaseous effluent supply being made to the upper portion of the contact zones along with the organic solvent supply.
In a second, preferred, variation, both the gas and organic solvent in the first contact zone and the gaseous effluent and organic solvent in the second contact zone are brought into contact in counter-current mode, the gas supply or the gaseous effluent supply being made to the lower portion of the contact zones and the organic solvent supply being made to the upper portion of the contact zones.
Clearly, the process can also be carried out in horizontal reactors.